# Coating thickness gauges

Selecting magnetic, eddy-current and ultrasonic coating measurements, with substrate effects, adjustment and reporting limits.

Reviewed: 2026-11-01

Tags: inspection, instruments, coatings

## Start with the coating and substrate

Choose the principle by coating, substrate and whether the drawing calls for a single layer or the complete coating system. [DeFelsko, How Is Coating Thickness Measured?](https://www.defelsko.com/resources/coating-thickness-measurement)

As an **original arithmetic example**, assume a procedure requires subtracting a 12 µm base-metal reading from a 150 µm coated reading. The reported corrected thickness is **138 µm**. If the electronic gauge has already been adjusted to account for that substrate effect, subtracting 12 µm again would report 126 µm and duplicate the correction. These assumed values illustrate recordkeeping, not a prescribed roughness allowance.

| Principle | Typical application | Important boundary |
| --- | --- | --- |
| Magnetic pull-off or magnetic/electromagnetic induction | Non-magnetic coating over magnetizable metal, commonly steel | Requires the appropriate magnetic response; not a general method for paint on plastic |
| Amplitude-sensitive eddy current | Non-conductive coating over non-magnetic, electrically conductive metal | Coating and substrate electrical properties matter |
| Ultrasonic pulse-echo | Suitable coatings over wood, plastic, concrete and other substrates | Requires an interpretable acoustic interface and appropriate coating sound velocity |

ISO 2178:2016 defines the magnetic method for non-magnetizable coatings on magnetizable base metals. Its third edition was published in **March 2016** and ISO records confirmation in 2026. ISO 2360:2017 identifies the amplitude-sensitive eddy-current method for non-conductive coatings on non-magnetic conductive base metals. These scopes help select a method; they do not prescribe a universal acceptable thickness. [ISO 2178](https://www.iso.org/standard/63004.html), [ISO 2360](https://www.iso.org/standard/69943.html)

## Magnetic and eddy-current instruments

Magnetic pull-off instruments infer thickness from the force needed to detach a magnet. Electronic magnetic instruments infer separation from a magnetic-field response. Eddy-current instruments infer thickness from the interaction between a probe field and the conductive substrate. Combination instruments can switch principles, but the operator must still confirm that the selected mode suits the actual coating/substrate pair. [DeFelsko, magnetic principles](https://www.defelsko.com/feature/magnetic), [eddy-current principle](https://www.defelsko.com/feature/eddy-current)

Do not select solely from “ferrous/non-ferrous” wording on the display. Elcometer's application matrix distinguishes specific combinations: paint over aluminum uses its NF probe, while paint over steel uses F. Some metallic coatings require a known sample for setup. Its matrix explicitly directs users to seek an application assessment for combinations not listed. [Elcometer, probe selection by coating and substrate](https://www.elcometer.com/en/what-is-the-correct-coating-thickness-gauge-probe-for-each-coating-substrate)

A simple gauge reading over a multilayer system is not automatically the thickness of its topcoat. Identify which physical interface the method detects. If layer-specific thickness is required, plan a suitable sequence of measurements during application or another validated method rather than assigning the total reading to the final layer.

## Ultrasonic measurement and its limits

The **PosiTector 200** manual describes sending a sound pulse through couplant into the coating, then converting its return travel time using coating sound velocity. Reflections can originate at coating/coating as well as coating/substrate interfaces. The instrument normally interprets the largest echo within the selected range unless another mode is selected. A stable number can therefore represent an unintended interface. The manual's back cover bears **©2024**; that is its printed copyright year, not a claimed calendar revision date. [DeFelsko, PosiTector 200 instruction manual, printed page 3](https://dl.defelsko.com/pdf/manuals/PosiTector200-Z-InstructionManual.pdf)

Couplant is required to transmit sound into the coating. The manual recommends checking that the couplant will not stain or damage the finish. Probe range, echo selection and the representative coating used for adjustment all belong in the application method. [PosiTector 200 manual, couplant and operation](https://dl.defelsko.com/pdf/manuals/PosiTector200-Z-InstructionManual.pdf)

A bounded model example is the **200 B1**, listed for polymer coatings on wood, plastic and similar substrates: range **13–1,000 µm**, with stated accuracy **±(2 µm + 3% of reading)**. At a 200 µm reading the expression evaluates to **±8 µm**. This arithmetic is not a measurement-uncertainty evaluation or a pass/fail decision. The manufacturer qualifies range limits as applying to polymer-based coatings and warns that aggregate-filled coatings can cause unreliable ultrasonic results. Advanced models' layer capability also depends on the materials being tested. [DeFelsko, PosiTector 200 specifications and application FAQ](https://www.defelsko.com/positector-200)

## Calibration, verification and adjustment

Keep three activities distinct:

- **Calibration:** a controlled, documented comparison with traceable standards under specified conditions.
- **Verification:** checking that the instrument meets the applicable performance requirement against a suitable reference.
- **Adjustment:** changing its response to improve agreement for a particular substrate or application.

DeFelsko's terminology distinguishes these activities and describes calibration as typically performed by the manufacturer or a qualified laboratory. A field zero or shim adjustment does not create a new laboratory calibration certificate. Conversely, a calibration certificate does not eliminate the need to check performance in the actual application. [DeFelsko, Calibration Terms](https://www.defelsko.com/resources/calibration-terms-coating-thickness-gages)

Use references suitable for the principle. DeFelsko permits plastic shims with electronic instruments but does not recommend them for mechanical magnetic pull-off gauges: shim curvature can act as a spring and disturb detachment. A shim's tolerance and condition also affect the check; nominal thickness printed on a worn or creased foil is not an exact reference value. [DeFelsko, Coating Thickness Standards](https://www.defelsko.com/coating-thickness-standards)

## Geometry, roughness and measurement conditions

Curvature, thin substrates, alloy properties, roughness and proximity to edges can change the response. Eddy-current testing is sensitive to substrate conductivity as well as these geometric conditions. Verify on a representative uncoated part or an appropriate reference arrangement; a flat smooth zero plate cannot by itself establish performance on a small curved component. [DeFelsko, measurement-method influences](https://www.defelsko.com/resources/coating-thickness-measurement)

For roughened steel, the effective magnetic surface lies between profile peaks and valleys. DeFelsko explains that a Type 1 reading can require subtraction of a base-metal reading, while an appropriately adjusted Type 2 instrument can indicate the corrected thickness directly. Follow the specified method and avoid applying the same correction twice. [DeFelsko, Calibration Terms: roughness](https://www.defelsko.com/resources/calibration-terms-coating-thickness-gages)

Its standards guidance describes taking several measurements on prepared, uncoated substrate and using their mean as the base-metal reading. Preserve access to a representative substrate before coating when the inspection method needs it. [DeFelsko, rough-surface adjustment](https://www.defelsko.com/coating-thickness-standards)

## Results, uncertainty and acceptance

Display resolution is the increment shown; a manufacturer's accuracy expression describes stated instrument performance under its applicable conditions. Measurement uncertainty must also account for references, adjustment, repeatability, substrate effects and the workpiece method. Report neither the smallest displayed increment nor a catalog percentage as the uncertainty without an evaluation.

ASTM **D7091-22** explicitly separates calibration, verification and adjustment. Its public scope also warns that single-point readings may not represent a whole coating system and assigns minimum/maximum layer and total-system thickness requirements to the governing specification. Use the contract's sampling and acceptance rules, including the applicable edition; do not invent one acceptable thickness for all coatings. [ASTM D7091-22, scope and significance](https://store.astm.org/d7091-22.html)

Record gauge and probe identification, method and mode, reference values, verification results, adjustments or corrections, substrate/geometry, measurement locations and the individual readings needed by the inspection plan. Keep individual readings distinguishable from spot or area averages so that a reported mean does not conceal a local nonconformance.

## Related reference

[Inspection and quality documents](../../sourcing/documents/index.md)